There is conventionally known a microwave plasma generator including a coaxial microwave cavity with a double-pipe discharge tube which consists of an outer tube and an inner tube and generating mixture gas plasma with microwave (refer to Patent Document 1). In the conventional microwave plasma generator, the inner and outer tubes are fixed to the cavity so as not to be displaced in the axial direction. Each of the inner and outer tubes is a straight tube. The cross section of a gap between the inner and outer tubes, that is, an opening space in a section orthogonal to the axis of the discharge tube is constant.
In the microwave plasma generator of this kind, the amount of active species such as radicals (free radicals) and ions depends on the degree of mixture of a carrier gas and a reactant gas supplied from the inner and outer tubes to the cavity. To generate a desired amount of active species such as radicals and ions, the flow rate and concentration of the gases and the energy of microwave applied to the gases have to be adjusted. In the conventional microwave plasma generator, however, it is difficult to perform the adjustment.
When the discharge tube with such a configuration is used, the efficiency of reaction between the carrier gas and the reactant gas is very low. To obtain plasma having the desired energy, a large amount of the carrier gas and the reactant gas is required.
[Patent document 1] Japanese Laid-Open Patent Publication No. 2000-133494